On the white dwarf distances to galactic globular clusters

¹ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany
³ Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy
⁴ Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Jordi Girona Salgado s/n, Módul B-4, Campus Nord, 08034 Barcelona, Spain
⁵ Institut d'Estudis Espacials de Catalunya
⁶ Institut de Ciències de l'Espai (CSIC), Edifici Nexus, Gran Capitá 2-4, 08034 Barcelona, Spain

Corresponding author: M. Salaris, ms@astro.livjm.ac.uk

Received: 11 January 2001
Accepted: 9 March 2001

Abstract

We analyze in detail various possible sources of systematic errors on the distances of globular clusters derived by fitting a local template DA white dwarf sequence to the cluster counterpart (the so-called WD-fitting technique). We find that the unknown thickness of the hydrogen layer of white dwarfs in clusters plays a non negligible role. For reasonable assumptions -supported by the few sparse available observational constraints -about the unknown mass and thickness of the hydrogen layer for the cluster white dwarfs, a realistic estimate of the systematic error on the distance is within ±0.10 mag. However, particular combinations of white dwarf masses and envelope thicknesses -which at present cannot be excluded a priori -could produce larger errors. Contamination of the cluster DA sequence by non-DA white dwarfs introduces a very small systematic error of about -0.03 mag in the plane, but in the plane the systematic error amounts to ~+0.20 mag. Contamination by white dwarfs with helium cores should not influence appreciably the WD-fitting distances. Finally, we obtain a derivative for the WD-fitting distances, which is very similar to the dependence found when using the Main Sequence fitting technique.